This disclosure relates to water reducible coating compositions, in particular water reducible paint compositions comprising a carboxy ester ketal, methods for the manufacture of the compositions, and uses of the compositions.
A variety of water-borne compositions for the formation of coatings are known, for example latexes, emulsions, and water-reducible compositions. “Latexes” refers to compositions containing a latex polymer binder in water. As is known in the art, a latex is a dispersion of sub-micrometer polymer particles formed by emulsion polymerization. The polymer backbone is formed from monomers capable of free-radical polymerization, such as (meth)acrylate monomers, optionally together with various ethylenically unsaturated co-monomers. Because the polymerization process for latexes utilizes emulsion polymerization technology, latexes are sometimes referred to as “emulsions” in the art. However, here, a latex is distinct from an “emulsion coating composition,” which refers to waterborne systems in which liquid binders are dispersed in an aqueous continuous phase. Emulsion coating compositions are less common than latex systems, but are utilized in some epoxy-amine, alkyd, and polyurethane products. The binders can include hydrophilic co-monomers to aid in dispersing the resin in the aqueous phase. Surfactants can also be used, alone or in addition to a co-monomer.
“Water-reducible coating compositions” are produced using traditional polymerization techniques rather than emulsion polymerization and often contain a water-miscible organic solvent. The polymer binder is usually modified to make it compatible with the water/organic solvent system. The modification can involve a hydrophilic co-monomer, such as an organic acid, that can impart water reducibility to the oligomeric or polymeric binders. Another modification is to disperse the binder in an aqueous phase with the use of a surfactant, which in some cases can be chemically incorporated into the polymer binder during synthesis of the binder. In each case, the resulting polymers are subsequently mechanically dispersed into the aqueous phase. When polymer particles (either liquid or solid) are formed in water, the water-reducible coating composition is an emulsion (for liquid polymers) or a dispersion (for solid polymers). Another version of a water-reducible coating composition is based on the polymer binder being soluble in the organic solvent/water mixture or partially soluble in the organic solvent/water mixture because the polymer chains form aggregates that are dispersed in the liquid aqueous phase. The solvent partitions between the polymer aggregates and the water. Thermosetting acrylics, epoxies, polyesters, epoxy-esters, and alkyds are examples of binders used in water-reducible coating compositions.
Polyurethane dispersions (PUDs) are examples of water-reducible coating compositions that are aqueous polymer dispersions. The most common method of preparing a PUD is to disperse a prepolymer in water and then build molecular weight by chain extension. The prepolymer can be formed from the reaction of a polyol, an ionizable co-monomer such as 2,2-dimethylolpropionic acid (DMPA), and excess diisocyanate to yield an isocyanate-terminated prepolymer. The DMPA units are converted to their ionic form (by neutralizing with a tertiary amine, for instance) and the prepolymer is diluted with water. The ionizable co-monomer improves the dispersibility of the polymer and helps to stabilize the polymer particle in water. An amine is added to chain extend the prepolymer, producing a high molecular weight polymer in water. Branching and crosslinking may be possible, depending on the choice of comonomers and chain extenders. N-methylpyrrolidone (NMP) is often used during PUD synthesis to dissolve DMPA and to reduce the viscosity in order to make dispersion easier. Another special case of water-reducible coating compositions include those based on alkyd polymer binders, which can have acid groups that are neutralized to allow dispersion in a water-reducible system.
While all water-borne coating compositions contain a polymer binder and water, the different properties of the binders result in different formulation requirements. As mentioned above, water-reducible compositions are often initially dissolved in a nonaqueous solvent to reduce viscosity and aid the dispersion in water. The compositions can further be formulated with a variety of additives, among them a coalescing solvent to promote film formation. The coalescing solvent softens the polymer binder particles by reducing the glass transition temperature (Tg) of the particles, and enabling them to fuse into a continuous film. Thus, when a water-reducible coating composition containing a coalescing solvent is coated onto a substrate, the coating cures by coalescence, where the water and the coalescing solvent evaporate sequentially or simultaneously. During evaporation, the coalescing solvent draws together and softens the polymer binder particles, fusing them together into an entangled polymer film. In some cases, the coalescent does not evaporate and can serve as a plasticizer for the final film. Chemical curing, through a crosslinking agent or an oxidative process, can occur after the film is substantially cured through the solvent evaporation mechanism.
Solvent selection for water-reducible coatings is guided by considerations such as solubility, reactivity, volatility, toxicity, environmental profile, and cost. While a number of solvents are available and in commercial use, there remains a need in the art for new solvents that offer a favorable combination of these characteristics. Further, there is an increasing desire for “bio-sourced” solvents that can be used as replacements for petroleum-sourced solvents. Few bio-source solvents are available that can meet the increasingly demanding technical requirements for water-reducible coating compositions and their resultant coatings, including paints. Even where such solvents are available, the solvents can have various drawbacks. For example, ethanol is a versatile solvent that is readily available from bio-based sources, but its high flammability limits its use in many applications. A further drawback of many bio-sourced solvents is that their chemical and physical properties can only be adjusted to a limited extent.